The overall objective is to characterize hyperplastic lesions of the rat mammary gland: in vivo-induced hyperplastic alveolar nodules, in vitro-induced hyperplastic alveolar nodules which arise spontaneously in noncarcinogen-treated animals. Mammary tumors will be characterized according to the hyperplasia or origin. Attempts will be made to enhance and inhibit tumorigenesis of the hyperplasias. The methodology will include mammary fat pad transplantation, organ culture, endocrinological manipulation, and morphology, both subgross and microscopic. Primary and transplantable hyperplasias will be studied in parallel.